1. Field of the Invention
The present invention relates to a tandem welding torch including a shock sensor unit attached to a robot arm of a welding robot that performs tandem welding with two welding torches.
2. Description of the Related Art
For example, Japanese Unexamined Patent Application Publication Nos. 2004-351510 (hereinafter referred to as PTL 1, paragraph [0006], FIGS. 1 and 2) and 2009-34746 (hereinafter referred to as PTL 2, FIGS. 2 to 4) each disclose a welding torch attached to a welding robot. A typical welding torch is equipped with a shock sensor unit (shock sensor) that protects the welding torch when the welding torch collides with a foreign object such as an object to be welded or a jig.
The shock sensor unit described in PTL 1 and PTL 2 is a single-torch sensor that stops the welding robot when it detects a collision of one welding torch (nozzle) with a foreign object. The shock sensor unit is interposed between a flange joined to a hollow wrist portion of the welding robot and the welding torch.
For example, Japanese Unexamined Patent Application Publication No. 2005-324250 (hereinafter referred to as PTL 3, FIG. 4) discloses a shock sensor unit (shock sensor) attached to a tandem welding torch. The shock sensor unit is provided at a distal end of an arm of a welding robot, and two welding torches are attached to a distal end of the shock sensor unit with a bracket, a torch clamp, etc. being disposed therebetween.
In the tandem welding torch described in PTL 3, one shock sensor unit is provided at the distal end of the arm of the welding robot, a first torch and a first connector are provided at a distal end of the shock sensor unit with a bracket being disposed therebetween, and a second torch is attached to the first connector with a second connector being disposed therebetween.
However, in the welding torch and the welding robot described in PTL 1 and PTL 2, one shock sensor unit is provided along a center line of one welding torch. Even when two units, each including a single welding torch and a single shock sensor unit, are juxtaposed, they are not applicable to a tandem welding torch including two welding torches.
That is, even if two welding torches, each including a shock sensor unit, are simply juxtaposed, they separately move in a collision with a foreign object to be welded. Hence, the distance between the distal ends of the welding torches mounted in position and orientations and mount positions of the welding torches are changed, and this makes accurate welding difficult. For this reason, the welding torch described in PTL 1 and PTL 2 is not applicable to a tandem welding torch.
The shock sensor unit attached to the tandem welding torch described in PTL 3 receives the total mass of the tandem welding torch, the bracket, and a torch cable and tensile and compression loads on the torch cable produced by robot operation. For this reason, it is necessary to use a strong spring member in order to obtain a great holding force for holding the tandem welding torch.
Accordingly, the detection sensitivity of the shock sensor unit having such a structure needs to be reduced in order to prevent erroneous detection when the tensile and compression loads on the torch cable are applied.
As a result, the shock sensor unit needs to hold the welding torch with a spring force more than a proper force. Since the spring force for holding the welding torch is great, it lays a heavy burden. The shock sensor unit does not operate unless a force greater than the spring force is applied thereto when the welding torch collides with an object to be welded, and therefore, the welding torch is bent.
Further, since the sensitivity to impact is set low, the shock sensor unit delays detecting the collision of the welding torch with the object. Therefore, the welding torch is deformed by the impact, or welding wires in the welding torch are misaligned with a welding line.
When the welding wires are misaligned, welding defects, such as an undercut and incomplete penetration, occur, and this deteriorates welding quality. For this reason, it is necessary to accurately align two welding wires with the welding line.
In the tandem welding torch described in PTL 3 and the tandem welding torch including two welding torches of PTL 1 and PTL 2, power is separately supplied to the two welding torches. Hence, in the shock sensor unit, it is essential to isolate two electrodes and to isolate the arms of the welding robot. This increases the number of components and complicates the structure and assembly operation.